Abstract: MicroRNAs (miRNA), a class of small non-coding RNAs, are emerging as important regulators of gene expression in the immune system by functioning as endogenous inhibitors of translational processes by binding to the 3'-UTR of target genes. There is increasing evidence that miRNAs function as an effective system to regulate the magnitude of inflammatory responses and have been associated with the pathogenesis of various human diseases. We have established recently that miR-10a, conserved in both mouse and human spp, was highly expressed in intestine dendritic cells (DC), and negatively regulated by microbiota through the interaction of TLR-TLR ligands via the MyD88 pathway. We identified IL-12/IL-23p40, which is shared by IL-12 and IL-23, two important cytokines in the pathogenesis of inflammatory bowel disease, as a target gene of miR-10a, in that miR-10a directly bound to the 3'-UTR of the IL-12/IL-23p40 gene, and overexpression of miR-10a inhibited DC IL-12/IL-23p40 expression and production of IL-12 and IL-23. miR-10a expression was decreased in the inflamed intestinal lesions in mice with colitis, which was correlated with high levels of IL- 12 and IL-23, compared to that in normal mice. A recent report further demonstrated that miR-10a attenuated regulatory T cell (Treg) conversion into effector T cells and inhibited Th17 differentiation. In the present proposal, our preliminary data demonstrate that miR-10a expression is decreased in intestinal biopsies of patients with Crohn's disease (CD) and ulcerative colitis (UC), when compared to its expression in normal individuals, a finding that is indicative of the relevance of dysregulated miR-10a to human IBD. NFkB activation is elevated in IBD patients and localizes primarily in the intestines to lamina propria dendritic and epithelial cells. Overexpression of miR-10a suppresses DC expression of NFkB activation and TNFa production and inhibits colitis development in an animal model. In this project, we will investigate the regulatory targets of miR-10a in mucosal DC, and the mechanisms of how miR-10a expression regulates mucosal DC activation and function in response to microbiota. Lastly, we will test our hypothesis that DC expression of miR-10a shifts mucosal T cell responses to low levesl of Th1/Th17 and high levels of Treg, which leads to preservation of intestinal immune homeostasis and prevention of inflammatory bowel disease.